The Great Sumatra-Andaman Earthquake of 26 December 2004

The two largest earthquakes of the past 40 years ruptured a 1600-kilometer-long portion of the fault boundary between the Indo-Australian and southeastern Eurasian plates on 26 December 2004 [seismic moment magnitude (M_w) = 9.1 to 9.3] and 28 March 2005 (M_w = 8.6). The first event generated a tsunami that caused more than 283,000 deaths. Fault slip of up to 15 meters occurred near Banda Aceh, Sumatra, but to the north, along the Nicobar and Andaman Islands, rapid slip was much smaller. Tsunami and geodetic observations indicate that additional slow slip occurred in the north over a time scale of 50 minutes or longer

Geodetic and seismic constraints on some seismogenic zone processes in Costa Rica

New seismic and geodetic data from Costa Rica provide insight into seismogenic zone processes in Central America, where the Cocos and Caribbean plates converge. Seismic data are from combined land and ocean bottom deployments in the Nicoya peninsula in northern Costa Rica and near the Osa peninsula in southern Costa Rica. In Nicoya, inversion of GPS data suggests two locked patches centered at 14 ± 2 and 39 ± 6 km depth. Interplate microseismicity is concentrated in the more freely slipping intermediate zone, suggesting that small interseismic earthquakes may not accurately outline the updip limit of the seismogenic zone, the rupture zone for future large earthquakes, at least over the short (∼1 year) observation period. We also estimate northwest motion of a coastal “sliver block” at 8 ± 3 mm/yr, probably related to oblique convergence. In the Osa region to the south, convergence is orthogonal to the trench. Cocos-Caribbean relative motion is partitioned here, with ∼8 cm/yr on the Cocos-Panama block boundary (including a component of permanent shortening across the Fila Costeña fold and thrust belt) and ∼1 cm/yr on the Panama block–Caribbean boundary. The GPS data suggest that the Cocos plate–Panama block boundary is completely locked from ∼10–50 km depth. This large locked zone, as well as associated forearc and back-arc deformation, may be related to subduction of the shallow Cocos Ridge and/or younger lithosphere compared to Nicoya, with consequent higher coupling and compressive stress in the direction of plate convergence

Seismogenic zone structure beneath the Nicoya Peninsula, Costa Rica, from three-dimensional local earthquake P- and S-wave tomography

The subduction plate interface along the Nicoya Peninsula, Costa Rica, generates damaging large (Mw > 7.5) earthquakes. We present hypocenters and 3-D seismic velocity models (VP and VP/VS) calculated using simultaneous inversion of P- and S-wave arrival time data recorded from small magnitude, local earthquakes to elucidate seismogenic zone structure. In this region, interseismic cycle microseismicity does not uniquely define the potential rupture extent of large earthquakes. Plate interface microseismicity extends from 12 to 26 and from 17 to 28 km below sea level beneath the southern and northern Nicoya Peninsula, respectively. Microseismicity offset across the plate suture of East Pacific Rise-derived and Cocos-Nazca Spreading Center-derived oceanic lithosphere is ∼5 km, revising earlier estimates suggesting ∼10 km of offset. Interplate seismicity begins downdip of increased locking along the plate interface imaged using GPS and a region of low VP along the plate interface. The downdip edge of plate interface microseismicity occurs updip of the oceanic slab and continental Moho intersection, possibly due to the onset of ductile behaviour. Slow forearc mantle wedge P-wave velocities suggest 20–30 per cent serpentinization across the Nicoya Peninsula region while calculated VP/VS values suggest 0–10 per cent serpentinization. Interpretation of VP/VS resolution at depth is complicated however due to ray path distribution. We posit that the forearc mantle wedge is regionally serpentinized but may still be able to sustain rupture during the largest seismogenic zone earthquakes

Intrusions and anomalous Vp/Vs ratios associated with the New Madrid seismic zone

Detailed P wave velocity (Vp) and S wave velocity (V s) models and Vp/Vs ratios were determined for a major portion of the New Madrid seismic zone using arrival times recorded by the New Madrid seismic network and Portable Array for Numerical Data Acquisition (PANDA) stations. We performed a simultaneous inversion for P and S wave velocities and hypocentral locations, yielding the most detailed tomographic image of the upper portion of the crust to date. Low Vp and high Vs anomalies resulted in low Vp/Vs ratios that correspond to the major arms of seismicity north of the intersection of the Cottonwood Grove-Blytheville Arch (CG-BA) fault with the Reelfoot fault. The unusual low Vp/Vs values can be attributed to the presence of quartz-rich rocks. Two regions contain anomalous Vp and V s values and Vp/Vs ratios that cannot be attributed to variations in rock composition and are probably produced by overpressured fluids. One region is located on the hanging wall of the northern portion of the Reelfoot fault and is aseismic. The other region corresponds to a portion of the southern Reelfoot fault that experiences swarm activity. A distinct velocity contrast exists across the CG-BA fault at depths exceeding 4.65 km; basement rocks southeast of the fault have Vp values that are 4%-6% slower than values for rocks located to the northwest. The most logical explanation is that the fault follows a preexisting structural feature or lithologic change in basement rocks. Copyright © 2010 by the American Geophysical Union

Mysterious tremor-like signals seen on the reelfoot fault, Northern Tennessee

A phased array of 19 broadband seismometers was deployed from November 2009 to September 2011 in an effort to detect nonvolcanic tremor or tectonic tremor associated with the Reelfoot fault, northern Tennessee. An autodetection algorithm using broadband frequency–wavenumber analysis was used to search for the recurrence of signals first reported during an active source experiment in 2006. The original signals appeared as short duration, impulsive arrivals with a high phase velocity ranging from 3 to 25 km=s.We have identified thousands of similar signals on the 2-year long array data. Two distinct detection peaks are observed with event azimuths from the west and northeast. The detections are most similar to the events seen in 2006 and are inferred to come from very small (ML ~ −1) microearthquakes that occur in the shallow basement on faults adjacent to the Reelfoot fault. These include detections with coherent S-wave energy that reinforce the interpretation of very small local and regional events. Other signals detected show distinct changes in slowness and azimuth as a function of time. These events were interpreted as atmospheric acoustic sources. The high-frequency content and impulsive arrivals of the nonacoustic arrivals are not consistent with tectonic tremor as seen in other parts of the world but do indicate seismic activity in the crust near the Reelfoot thrust fault that was previously unknown

Investigating the P wave velocity structure beneath Harrat Lunayyir, northwestern Saudi Arabia, using double-difference tomography and earthquakes from the 2009 seismic swarm

In 2009, a swarm of more than 30,000 earthquakes occurred beneath the Harrat Lunayyir lava field in northwest Saudi Arabia. This event was just one of several seismic swarms to occur in this region over the past decade. Surface deformation associated with the seismicity, modeled in previous studies using Interferometric Synthetic Aperture Radar (InSAR) data, is best attributed to the intrusion of a 10 km long dyke. However, little is known about the velocity structure beneath Harrat Lunayyir, making assessment of future seismic and volcanic hazards difficult. In this study, we use local double-difference tomography to generate a P wave velocity model beneath Harrat Lunayyir and to more precisely locate earthquakes from the 2009 seismic swarm. A pronounced fast velocity anomaly, centered at ∼15 km depth with a shallower extension to the N-NW, is interpreted as an area of repeated magmatic intrusion. The crust surrounding the fast intrusion is slower than that suggested by broader-scale models for the Arabian Shield. The largest magnitude events occurred early in the swarm, concentrated at shallow depths (∼2-8 km) beneath northern Harrat Lunayyir, and these events are associated with the dyke intrusion. Later, deep earthquakes (∼15 km) beneath the southern end of the study region as well as a group of intermediate-depth events connecting the shallow and deep regions of seismicity occurred. These later events likely represent responses to the local stress conditions following the intrusion. Our results are unique since harrat magma systems are rarely imaged, and our observations, coupled with the seismic history in this region, suggest that future volcanic intrusions beneath Harrat Lunayyir are likely. Key PointsFast velocities beneath Harrat Lunayyir are interpreted as magmatic intrusionsCrustal velocities are slower than those suggested by broader-scale modelsEarthquakes from the 2009 swarm delineate the orientation of dyke intrusion © 2013. American Geophysical Union. All Rights Reserved

SEISMOGENIC ZONE STRUCTURE ALONG THE MIDDLE AMERICA TRENCH: RESULTS FROMTHE COSTA RICA SEISMOGENIC ZONE EXPERIMENT

Presented before the 2003 IUGG General Assembly, Sapporo, Japan, June 30-July 11, 2003.IUGG General Assembl

Along-strike variability in the seismogenic zone below Nicoya Peninsula, Costa Rica

OVSICORIAt the subduction zone in northwestern Costa Rica, the seismogenic zone lies directly beneath the Nicoya Peninsula, allowing for near source seismic studies of earthquake activity. We located 650 earthquakes along the seismogenic plate interface using a dense seismic network in the vicinity of the Nicoya Peninsula. Using these data we constrained the updip limit of the seismogenic zone there and found a transition in depth, 10 km in the south to 20 km in the north, that occurs where the subducting oceanic crust changes from warmer Cocos-Nazca Spreading center (CNS) origin to colder East Pacific Rise (EPR) origin. We argue that the temperature of the incoming oceanic crust controls the seismogenic updip limit beneath Nicoya, Costa Rica; subducting colder oceanic crust deepens the seismogenic updip limit.En la zona de subducción en el noroeste de Costa Rica, la zona sismogénica se encuentra directamente debajo de la península de Nicoya, lo que permite estudios sísmicos de fuentes cercanas a la actividad sísmica. Localizamos 650 terremotos a lo largo de la interfaz de placas sismogénicas utilizando una red sísmica densa en las cercanías de la Península de Nicoya. Usando estos datos, restringimos el límite echado arriba de la zona sismogénica allí y encontramos una transición en profundidad, de 10 km en el sur a 20 km en el norte, que ocurre donde la corteza oceánica en subducción cambia desde el centro de expansión Cocos-Nazca más cálido (CNS) origen al origen más frío de la Dorsal del Pacífico Oriental (EPR). Argumentamos que la temperatura de la corteza oceánica entrante controla el límite de buzamiento ascendente sismogénico debajo de Nicoya, Costa Rica; la subducción de una corteza oceánica más fría profundiza el límite de buzamiento ascendente sismogénico.University of California Santa Cruz, USAUniversidad Nacional, Costa RicaUniversity of California, USAObservatorio Vulcanológico y Sismológico de Costa Ric

A community experiment to record the full seismic wavefield in Oklahoma

Observing the full seismic wavefield by deploying large numbers of seismometers (also known as large-N deployments) and analyzing the resultant large datasets is now more feasible than ever before as a result of advances in instrumentation, computational power, and data analysis techniques. In 2015, the Incorporated Research Institutions for Seismology (IRIS) proposed a field deployment to provide the research community with experience in new techniques for obtaining full wavefield observations using a range of instrumentation (threecomponent [3C] nodal-style sensors, broadbands, and infrasound) at multiple spatial and temporal scales. The goals of the experiment were to demonstrate the field use of the nodal sensors, contribute a compelling dataset that could be analyzed through innovative techniques, and evaluate the performance of new array designs and instruments (particularly the 3C nodes). The resulting IRIS Wavefields Demonstration Community Experiment, conducted in north-central Oklahoma during the summer of 2016, collected data that were immediately made open and available at the IRIS Data Management Center (network code YW) and provided a unique and scientifically rich dataset to advance our understanding of the full seismic wavefield. A key finding was that by burying the 3C nodal sensors used in the deployment, substantially lower horizontal noise levels were achieved across a wide range of periods spanning 0.01-100 s